1. Field of the Invention
The invention relates to a vehicle power steering system that assists a driver's steering operation.
2. Description of the Related Art
There is a power steering system that assists a driver's steering operation by applying torque generated by an electric motor to a steering mechanism of a vehicle. A conventional power steering system of this type is described in Japanese Patent No. 4453012.
The power steering system includes a torque sensor that detects a steering torque applied by a driver, and a steering angle sensor that detects a steered angle of steered wheels. The power steering system sets a first assist torque component, which is a basic component of an assist torque, on the basis of the steering torque detected by the torque sensor. An ideal model that outputs an ideal target steered angle corresponding to the steering torque is provided in advance in the power steering system.
The power steering system sets a target steered angle from the steering torque detected by the torque sensor, on the basis of the ideal model, and obtains a second assist torque component that should be applied to the steering mechanism in order to maintain an actual steered angle detected by the steering angle sensor at the target steered angle. The power steering system assists a steering operation by executing assist control for applying the assist torque based on the first assist torque component and the second assist torque component from the electric motor to the steering mechanism. With such a configuration, even when some kind of disturbance is input into the steering mechanism, the steered angle is maintained at a target value. Thus, vibrations of the steering mechanism due to the disturbance are reliably suppressed. As a result, the driver's steering feel is improved.
Some vehicle power steering systems execute so-called lane keeping control for assisting a driver's steering operation by changing the assist torque that is applied to the steering mechanism such that the vehicle easily keeps travelling in a lane. If such lane keeping control is executed by the power steering system described in Japanese Patent No. 4453012, it is necessary to execute both the assist control and the lane keeping control with the use of the electric motor. Therefore, there is a possibility that the assist control and the lane keeping control will interfere with each other and the steering feel that is supposed to be obtained by the assist control will deteriorate.
Such a possibility may arise not only in the lane keeping control but also in a case where the power steering system described in Japanese Patent No. 4453012 executes control of setting an ideal steered angle on the basis of the travelling state of the vehicle and assisting a steering operation such that the actual steered angle coincides with the ideal steered angle, such as intelligent parking control for assisting a driver's steering operation during parking.